Four Apple employees have filed a patent for placing PV cells under the LCD touch screen inside a range of portable devices, including iPhones, iPods, PDAs and handheld GPS systems.

It’s not the first attempt to power talk time using the sun. Solar chargers for mobile devices have been on the market for a while now, and telecom players including Vodafone have said they intend to introducesolar-powered chargers for their products.

But, so far, the only phone that allows users to juice up their cell phones in the sun without using a charger is from HiTech Wealth, which began selling a solar phone for a pricey $510 last year.

The Apple patent could signify that this “fringe” technology could be coming to mainstream culture. After all, the company has proven its ability to transform micro fads into industry-changing trends.

Still, Apple acknowledges in its patent filing the inherent difficulties for such a design on small screens.

The solar power in a panel is directly proportional to its surface area, and the screen areas in portable devices tend to be small to leave space for buttons and display information. The iPhone and iPod have large screens that occupy most of the front of these devices, allowing more solar cells to be stacked beneath the display.

The patent calls also for connecting cells in a pattern that would allow some of the solar cells to continue to function even while others are blocked from sunlight when, for example, the user is gripping the device.

One hour of clear sun could power an iPhone for 25 minutes of talk time or enable you to listen to "Pocketful of Sunshine" by Natasha Bedingfield on your iPod for up to an hour.

But don’t throw away your charger just yet. It takes 10 hours to fully charge a device – more time than even the most devoted talkers spend outside.